Subject matter of the present invention is a process for removing cyanide from cyanide-contaminated soils and a method of qualitative analysis of cyanide in potentially or actually cyanide-contaminated soils based thereon.
In the course of the discovery of contaminated locations, for example by establishing land registers of places polluted in the past, but also upon occurrence of new contaminations of the soil by industrial accidents, cyanide-contaminated soils are encountered to an increasing extent. Mainly these contaminations have been caused by galvanization and electroplating plants: however this problem is also encountered in all places where larger amounts of cyanides have been handled.
In the past the problem has been solved by digging such contaminated soils out and committing them to toxic waste dumping grounds. This procedure is extremely expensive and difficult to carry out to an increasing degree, since the space available in dumping grounds for toxic waste is extremely restricted and will be even less available in the future. The problem becomes even more difficult and costly, if the involved soil is under or immediately adjacent to buildings, since digging the soil out may endanger the structural safety of the buildings. Thus, in extreme cases even a demolition of the respective buildings had to be contemplated.
Thus, the first object was to develop a process whereby the cyanide could be removed from contaminated soils, where a removal on-site or off-site could be useful in order to avoid the transportation to the toxic waste dumping ground. However, preferably the process should be feasible in situ as well in order to avoid the costs of excavation and the exposure to structural danger to the adjacent buildings.
It has been known to detoxify cyanide waste waters according to various methods, all of which, however, employ relatively concentrated solutions, and wherein the processes can be carried out in more or less sealed containers. Thus, for example, there has been known the detoxification with hypochlorite wherein, in an exothermic reaction, first cyanogen chloride is formed which is also toxic and which can be hydrolyzed at pH values in excess of 12 and be converted into cyanate and finally ammonium carbonate. In addition, this process is a problem if in the waste water there are present not only free alkali cyanides but also heavy metal cyanide complexes, since the latter prevent or at least severely slow down the reaction of the cyanide. Further known is the detoxification with perchemicals, and more specifically with Caro's acid or H.sub.2 O.sub.2. These procedures are particularly well suited for relatively concentrated cyanide solutions. Since at room temperature the reaction with hydrogen peroxide proceeds only very slowly the reaction must be carried out either at an elevated temperature or by using catalysts. As the catalysts there are added especially copper salts in amounts of 0.2 g/l of copper sulfate.
In two further processes, formaldehyde and H.sub.2 O.sub.2 or ozone are introduced. Both processes will work only at relatively high concentrations, are relatively expensive and are also not free from problems because of the toxicity of the chemicals.
A further known process is the oxidation with the oxygen of the air in the carbocyanide process. Therein, the cyanide solution is passed through a column charged with certain kinds of activate carbon, while at the same time air is introduced. In this process, part of the cyanide is reacted to form cyanate, and part of the cyanide is reacted to form nitrogen. The cyanate may be subsequently saponified to give ammonia and carbonic acid. This catalytic oxidation is only incompletely accomplished with nickel complexes and cannot be successfully achieved at all with iron complexes.
Also known is the oxidation of hydrocyanic acid in the gaseous state, and more specifically in the case of concentrated cyanide solutions. In the course thereof, the concentrated cyanide solution is reacted with strong acids--optionally waste acids as well. The liberated hydrocyanic acid is then stripped from the solution with a carrier gas and is passed to the oxidation. This process can be carried out only in a closed system and under greatest precautions. It has so far not proven to be useful in practice.
Hydrolysis of cyanides has already been carried out under catalytic conditions by passing the mixture of hydrocyanic acid and air over a noble metal contact, at which combustion occurs of hydrocyanic acid to produce carbon dioxide, nitrogen and water. In the presence of a less than stoichiometric amount of oxygen, carbon monoxide, nitrogen and water are formed as well. The heat of combustion of this process may be simultaneously employed for heating the solution to be detoxified. This process certainly is also not suitable for the detoxification of contaminated soils. Also hydrolysis of cyanide in an autoclave at temperatures of from 180.degree. C. to 200.degree. C. for the removal of salts used in hardening operations is not feasible for a detoxification of soils.
The removal of cyanides by precipitation as Berlin White or Berlin Blue makes it necessary to dispose of the precipitation products in a special waste dumping ground or to consign them to a specialized plant for the workup. Thus, these processes certainly are also not suitable to work up cyanide contaminated soils. A compilation of all of these techniques is found in company brochures by the company Degussa, "CYANIDE, Entgiftung cyanidischer Abwasser--eine Ubersicht uber gangige Verfahren" as well as "CYANIDE--Entgiftung cyanidischer Abwasser mit Persauerstoff-Verbindungen".